The present invention relates generally to a conveyor belt and a method of making the same and, more particularly, to a conveyor belt capable of withstanding increased belt tension and exhibiting an extended belt life.
One of the most commonly used types of conveyor belts for carrying diverse products along both straight and horizontally curved paths is a grid conveyor belt. Examples of conventional grid conveyor belts are the Cam-Grid(copyright) and the heavy duty Cam-Grid(copyright) conveyor belts available from Cambridge, Inc., the assignee of the present invention. A grid conveyor belt is shown generally in FIGS. 1 and 2 by reference numeral 100. Conveyor belt 100 includes a plurality of spaced transverse rods 110 slidably interconnected by at least two rows of U-shaped connecting links 120 disposed respectively along the inner and outer edges of the rods. The terminal ends of the transverse rods 110 are formed into enlarged heads or button heads 130 which retain the links on the rods and welds 140 are then provided to secure the link to the button head and to the rod, thereby preventing rotational movement of the links on the transverse rods. Alternatively, bar links may be provided between the button head and the U-shaped connecting link such that the weld therebetween is eliminated. The connecting links are disposed in a nested relationship relative to one another with slots being provided in the links in order to slidably receive the transverse rods. Although not illustrated, the transverse rods may also be provided with an overlay which can be used to support smaller products which may slip between the spacing of the rods or which must lay flat during the conveying operation.
Grid conveyor belts of this type have met with overwhelming market approval because of their ability to travel in straight line conveyor paths as well as in curved conveyor paths, thus making grid conveyor belts ideal for use on spiral cage conveyors. In a spiral cage conveyor, the belt travels in a helical or spiral direction and is driven by cage bars disposed within the center of the spiral which engage the inner edge of the conveyor belt. Despite their popularity and commercial success, however, grid conveyor belts are prone to a number of operational problems which result in undesirable performance characteristics and may ultimately lead to complete belt failure. Examples of such operational problems include weld fatigue, tenting, racking, buckling, and shug, each of which is further discussed below.
Referring to FIG. 2, the cage bars 150 of a spiral conveyor move slightly faster than the speed of the conveyor belt 100 in order to lower the tension in the belt. As a result, all of the belt and product weight is transmitted to the outer edge of the belt which is not collapsed. The outer edge links and the welds thereon are thereby placed under increased tension. Further, because the cage bars 150 are moving slightly faster than the conveyor belt 100, the bars 150 tend to push the inner edge links and the welds thereon into slight compression.
The cyclic nature in which the tension is caused in the conveyor belt leads to a variety of possible causes for belt failure. More specifically, while the conveyor belt is running in a straight travel path, the tension in the conveyor belt is evenly distributed to both the inner and the outer edges. As the belt moves into a curved path, however, the outer edge of the conveyor belt receives a much greater portion of the belt tension while the inside edge is put under a slight compression or tension. Each time this situation arises, that is, each time the conveyor belt transitions between a straight path and a curved path, the welds between the transverse rods and the links pass through a xe2x80x9cduty cycle.xe2x80x9d Repeated duty cycles of this nature may eventually cause weld fatigue, which generally occurs first on the outer edge links where the higher magnitude of tension occurs and which is followed by weld fatigue of the inner edge links.
Referring also to FIG. 3, the conventional weld 140 between the connecting link and the transverse rod is subjected to a variety of forces during conveyor belt travel. That is, the weld is subjected to tension forces denoted by arrows A which represent the tension transmitted through the connecting link to the welded joint, and by torque and shear forces denoted by arrows B which represents the torque force encountered from the drive sprockets. The resulting combination of these forces may result in weld fatigue such that eventually the welded attachment point cracks, i.e., there is a weld failure.
While weld fatigue in itself may not significantly impede the operation of the conveyor system, it may be a precipitating factor for other belt problems, such as belt tenting. Belt tenting can be described as the connecting links bending upwards out of the travel plane of the conveyor belt. When the conveyor belt collapses into a turn, compressive forces act to collapse the inner edge links and thereby allow the belt to traverse the curved path. However, without the presence of secure welds to restrict the rotational movement of the link relative to the rod, the links may raise out of the travel plane of the conveyor belt and form an upstanding tent. Tenting may cause undesirable movement and damage to the conveyed product, as well as a complete failure of the conveying system if machinery components should become jammed as a result of the raised or tented connecting links.
In addition to the welds creating a potential source for system failure, the structural limitations of the transverse rods may also lead to a variety of belt problems such as, for example, racking and buckling, which are a result of the rods flexing when they are driven. Racking occurs when the outside edge of the conveyor belt leads the inner edge of the conveyor belt, or vice versa, due to the increasing tension in the conveyor belt exceeding the bending strength of the transverse rod. Thus, as the conveyor belt traverses either a drive sprocket, idler roll, or the like, it may cause damage to the rods, overlays, and/or sprockets, in addition to causing damage to the conveyed product and misalignment of the conveying system. A further condition which may arise when the strength of the transverse rod is exceeded by the tension in the conveyor belt is buckling. Buckling generally involves a bending of the transverse rod in one or more places when the rods are pushed into the cage bars of a spiral conveyor or into dead curves because of the increased tension in the conveyor belt. Buckling may cause rod failure and product damage, and in the worse case, it can also lead to structural jamming and failure of the conveying system.
Finally, although not directly related to the failure of any particular conveyor belt component, grid conveyor belts may also suffer from the phenomenon of belt shug. Since the connecting links of a conveyor belt are disposed in a nested relationship relative to one another, an internal clearance is produced between the mating links when in the extended position. As shown in FIG. 4, the accumulated lateral clearance of the conveyor belt, commonly referred to as shug and shown by the reference xe2x80x9cGxe2x80x9d, can cause the belt to run in a crooked path rather than the desired straight path. This especially creates problems with edge drive units, such as spiral cage conveyors, and can also cause potential problems in product movement, vibration, and/or damage to the conveying system and product.
In order to overcome these disadvantages, the present invention provides a conveyor belt having a plurality of pivotal transverse rods extending laterally across the belt, with the transverse rods having inner and outer ends along inner and outer edges of the belt; a plurality of connecting links arranged in at least one longitudinal row along the inner and outer edges of the belt and pivotally interconnecting the inner and outer ends of the transverse rods, respectively, with each of the connecting links having a pair of openings extending laterally therethrough such that the transverse rods extend through the openings; and a weld connection between the rods and the connecting links, each of the weld connections including only a weld connection point between a trailing side of the transverse rod and the connecting link.
The present invention also provides a method of manufacturing a conveyor belt including the steps of providing a plurality of transverse rods, providing a plurality of connecting links having a first pair of openings extending laterally therethrough and a second pair of openings extending therethrough, arranging the transverse rods through the first and second pair of openings so as to pivotally interconnect the transverse rods in a longitudinally spaced arrangement, and welding the connecting links to the transverse rods at the first pair of openings on only a trailing side of the transverse rod.